Exercise tolerance remains impaired following cardiac transplantation. Chronotopic incompetence is believed to be the limiting factor. To investigate the importance of heart rate response to peak exercise capacity we plan to: in new tx's determine whether electrical connection between the innervated native SA node and dennervated transplanted SA node via atrial-atrial pacing improves exercise performance and monitor native SA node activity during exercise via esophageal electrode in those greater than 1 yr. post transplant-monitor.